Re: Interresting thread in comp.lang.eiffel

["Ken Garlington" <Ken.Garlington@computer.org>]

"Joachim Durchholz" <joachim dot durchholz@halstenbach.com> wrote in message
news:8kl25k$2q7k0$1@ID-9852.news.cis.dfn.de...
> Ken Garlington <Ken.Garlington@computer.org> wrote:
> > "Joachim Durchholz" <joachim dot durchholz@halstenbach.com> wrote:
> > > In other words: had DbC been applied not only to the software but
> also
> > > the the software-IRS interface,
> >
> > What do you mean by a software-IRS interface?
>
> The IRS is a piece of hardware from the point of view of the main
> controlling software of the Ariane, right?
>
> > And why is this interface relevant to the Ariane 5 disaster?
>
> The system crashed because the IRS flooded that interface with
> meaningless data, right?

Going back to your original statement, which was:

> In other words: had DbC been applied not only to the software but also
> the the software-IRS interface, then the programmer doing the
> integration would have seen the contract and would have had a chance to
> see the inconsistency between IRS specification and flight trajectory
> data.

Assuming that the IRS-side contract would have been with respect to range,
as in:

-- the contract from the IRS side
IRS_write_value (horizontal_bias: INTEGER) is
      require
         horizontal_bias <= 32767
      do
         ...

      ensure
         written_value <= 32767

      end

and given what the OBC would expect, as in:

OBC_read_value (void) return INTEGER is
  require
  do
     ...
  ensure
       read_value <= 32767
  end

This certainly wouldn't denote any inconsistency as far as I can tell,
particularly since on a MIL-STD-1553 bus it's physically impossible to
violate this contract!

So, once again, you can't simply assert the magic of DbC in the context of
Ariane 5. You have to (for a start) give a specific contract that would have
made it clear that there was a problem. After all: "the official report is
clear and detailed enough to enable software professionals, aerospace
experts or not, to form their own judgment."